weird yield stuff

using System;
using System.Collections.Generic;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main()
        {
            try {
                var upperFoo = MakeUppercase(null);

                Console.Write("Uppercased!");
            }
            catch (ArgumentNullException)
            {
                Console.Write("Exeption");
            }
        }

        public static IEnumerable<char> MakeUppercase(string foo)
        {
            if (foo == null)
                throw new ArgumentNullException("foo");

            foreach (var chr in foo)
                yield return Char.ToUpperInvariant(chr);
        }
    }
}